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When turning my head during dogfights the P51 and Spitfire both seem to violent flip/roll completely inconsistent with control inputs. It is particularly noticeable when turning my head during high G turns.
It feels as if turning beyond 90 degrees inverts the controls or such which is completely nauseating and of course terrible for the fight outcome due to loss of inertia.

I used to run TrackIR on these planes without issue. I have tried resetting all controls/config but to no avail. Strangely it doesn't occur in the F15, A10C etc. It does also occur on the spitfire.

I searched but didn't see anything similar. - has anybody experienced something like this or have any suggestions?

No nothing similar as you describe but I have had similar experiences that were traced to some axes mapped to unwanted additional inputs. DCS is a right pain in assigning axis all over to all sorts of devices - maybe check for unwanted assignments?